Nanotechnology Spotlight

(Nanowerk Spotlight) The lithium-ion batteries that you find in many of your electronic gadgets, like your smartphone, typically consist of two electrodes connected by a liquid electrolyte. Apart from being prone to problems, including leakage, low charge retention and difficulties in operating at high and low temperature, this liquid electrolyte makes it difficult to reduce the size and weight of the battery.

Finding low-cost solid materials capable of efficiently and safely replacing liquid electrolytes in these batteries has been a considerable research interest over the past years.

Of the various types of solid electrolytes that have been developed so far, composite polymer electrolytes exhibit acceptable Li-ion conductivity due to the interaction between nanofillers and polymer.

Composite polymer electrolyte is typically a mixture of ceramic nanofiller and polymer electrolyte. The nanoscale ceramic fillers are known to be able to enhance mechanical or thermal stability as well as ionic conductivity of polymer electrolyte.

"Unfortunately, conventional composites suffer from agglomeration of nanofillers at high weight ratio which deteriorates the distribution of fillers and results in discontinuous ionic conduction pathways," Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute, University of Texas at Austin, explains to Nanowerk. "In our recent study, we fabricated a three-dimensionally (3D) interconnected framework consisting of ceramic nanofillers via a facile templating method using nanostructured hydrogels. The 3D ceramic framework can prevent aggregation of nanofillers and provide a continuous conduction pathway resulting in excellent ionic conductivity and stability."

The conventional method for making composite polymer electrolytes is to physically mix nanoparticles with polymer electrolyte. The problem here is that the filler ratio is limited to only 10∼20wt%. At higher weight ratios the nanoparticles tend to agglomerate, resulting in deterioration of percolation and poor ionic conductivity.

"To solve this issue, we fabricated a pre-percolated network of ceramic filler instead of distributing particles in polymer," Yu notes. "In our study, a 3D interconnected ceramic framework provides continuous pathways for ion conduction. We believe that our novel method will help to develop composite materials in a different but much improved way than conventional particle distributions."

Illustration of fabrication procedure of 3D framework based composite polymer electrolyte. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)

The 3D interconnected ceramic filler ensures a continuous interphase between polymer matrix and ceramic filler where fast ion transport can be achieved. Interestingly, the weight ratio of this novel ceramic filler (40wt%) is much higher than typical particle-composite (10∼20wt%).

By preventing the agglomeration of particles, the researchers were able to achieve a higher degree of percolation, resulting in superior ionic conductivity.

The high weight ratio of the inorganic filler also enhances electrochemical and thermal stability of the composite polymer electrolyte.

In addition, the combination of nanoscale ceramic filler and organic polymer also provides sufficient mechanical integrity and flexibility.

Conductivity vs volume fraction of nanofiller. A typical particle filler shows agglomeration at high volume fraction (or high weight ratio) while the 3D framework in this work shows superior ionic conductivity even with higher volume fraction due to the percolated network. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)

"The enhanced ionic conductivity as well as superior electrochemical and thermal stability in our study may be of broad interest to many energy applications, where both high performance and stability are required," Yu points out. "We believe that the advantages of our composite polymer electrolytes will open the way for promising electrolyte structure designs for solid-state lithium-ion batteries."

Next, the team plans to extend the concept of 3D interconnected frameworks to other composite materials.

"Nanostructured hydrogel derived frameworks have the ability to tune the structure and material by simply changing starting precursors," says Yu. "It is also interesting to apply this structural design to other applications, for example composite electrodes."

"Creating composites with different phases requires a deep understanding of the interactions between the inorganic filler and the polymer matrix as well as the structural design," he concludes. "We are also looking more at this research direction by examining the mechanisms of ion transport and interfacial property design for energy storage systems based on different types of ceramic fillers and polymer matrices. It is also important to examine the scalability of this interconnected framework structure in order for it to be used in practical applications."